It has been known that a polyolefin is generally obtained by polymerizing olefin monomers such as ethylene and propylene using a Ziegler-Natta catalyst system containing a transition metal halide as a main catalyst component.
Ziegler-Natta catalyst systems having various characteristics have been recently known, and polyolefins produced by using the various Ziegler-Natta catalyst systems may exhibit diverse properties in terms of stereoregularity, molecular weight distribution, impact strength, melt-flow ability, rigidity, heat seal ability, isotacticity, or the like.
Meanwhile, organosilane compounds have been used in the Ziegler-Natta catalyst system as an internal electron donor upon producing a solid main catalyst component comprising a halogen-containing titanium compound supported on an activated magnesium dihalide compound or as an external electron donor introduced into a reactor in combination with a solid main catalyst component and aluminum-alkyl cocatalyst upon polymerization. Typically, the organosilane compounds used in the Ziegler-Natta catalyst system have Si—OR, Si—OCOR or Si—NR2 group (herein, R is alkyl having 1 to 20 carbon atoms, alkenyl, aryl, arylalkyl or cycloalkyl). Such organosilane compounds are described in U.S. Pat. Nos. 4,180,636, 4,242,479, 4,347,160, 4,382,019, 4,435,550, 4,442,276, 4,473,660, 4,530,912, and 4,560,671, in which they are used as internal electron donors, and in U.S. Pat. Nos. 4,472,524, 4,522,930, 4,560,671, 4,581,342, 4,657,882 and European patent application Nos. 45976 and 45977, in which they are used as external electron donors.
In particular, U.S. Pat. Nos. 4,784,983 and 4,861,847 disclose a catalyst system for use in olefin polymerization and copolymerization, consisting of (A) a solid product consisting essentially of titanium, magnesium, halogen, polycarboxylic acid esters and organic phosphorus compounds, (B) an organic aluminum compound, and (C) an organic silane compound; U.S. Pat. Nos. 4,990,479 and 5,438,110 disclose a catalyst system for olefin polymerization formed from (A) a solid titanium catalyst component containing magnesium, titanium and halogen as essential ingredients, (B) an organoaluminum compound, and (C) an organosilicon compound containing a cyclopentyl group, a cyclopentenyl group, a cyclopentadienyl group or a derivative derived from any of these groups; and U.S. Pat. No. 4,829,038 discloses a catalyst system for olefin polymerization comprising a solid, hydrocarbon-insoluble, magnesium-containing, titanium-containing, electron donor-containing component, an alkyl aluminum compound, and organosilane compound selected from the group consisting of diisobutyldimethoxysilane, diisopropyldimethoxysilane, t-butyltrimethoxysilane, di-t-butyldimethoxysilane, and mixtures thereof.
Further, Korean Patent Application No. 10-2005-0013131 discloses a Ziegler-Natta catalyst system for olefin polymerization to improve polymer properties using organosilicon compounds such as dicyclopentyldimethoxysilane, propyltriethoxysilane, and methylcyclohexyldimethoxysilane as a selectivity control agent (SCA). WO 95/21203 recognizes dominating behavior for SCA's, when used together in a single reaction step at molar ratios of SCA:transition metal of 33:1.
Korean Patent Application No. 10-2006-0078834 discloses a catalyst system for olefin polymerization to improve melt-flowability by mixing organosilicon compounds such as dicyclopentyldimethoxysilane and trioxasilocane, but there is a problem in that the catalytic activity is reduced.
If organosilane compounds are contained in the Ziegler-Natta catalyst system as an electron donor for the polymerization of olefin monomers having 3 or more carbon atoms such as propylene, an increase in the isotacticity of the resulting polymer is promoted. However, if upon Ziegler-Natta polymerization, organosilane compounds are used to promote an increase in the isotacticity of polyolefin, other characteristics including catalytic activity tend to reduce. Therefore, there is a need to develop a Ziegler-Natta catalyst system capable of improving various characteristics as well as utilizing organosilane compounds, and a related method.